In baseball, you can have a 100-game minor league season no problem. There are 25 players on the each team. If you average 5k attendance in some place like Tucson or Corvallis or Provo, you can break even.

In football, you need at least 50 players on each team. Because of the physicality of the game, you can't have more than 20 per season. Even if the minor league football team averages 20k fans at the games, it won't break have the same economics as the minor league football team. Attendance needs to get up to 50k or so. So how do you ignite that much excitement for the minor league team in a medium-sized city? Attach it to the local college that half the town is connected to in one way or another!

Minor league baseball is able to persist because the major league teams pay the salaries of many of the players through the farm system. At a minimum, the same would need to happen for minor league football. This league doesn't have that.